For conventional two-stroke combustion engines it is well known how to control the air/fuel mixture, but a high level of unburned hydrocarbon emissions is produced because in this engine the scavenging process is performed exclusively by a mixture of air and fuel. Some of the air/fuel mixture mixes with the exhaust gases and thereby some unburned fuel is lost out with the exhaust gases.
Two-stroke combustion engines using the additional air technique during the scavenging are previously known in the art. These engines reduce fuel consumption and exhaust emissions. But for these prior art engines it is a problem to control the air/fuel mix during normal operation and during start. One common start procedure for a conventional two-stroke engine is with a closed choke valve and a fully open throttle valve. In combination with a “conventional” additional air technique this will provide too much air for optimal starting conditions. Since these kind of engines are used in many different environments and subjected to troublesome conditions it is desired to achieve a robust operation. One issue with engines using the additional air technique is to also provide good start properties. The operational conditions for this kind of engines can be of varying temperature, humidity, atmospheric pressure etc.
Hence, there is a need for an improved system for a two-stroke engine using the additional air technique to achieve good properties for start and normal operation. Further it is an advantage to combine proper operation of such an engine with lean and environmentally friendly operation. Finally it would be advantageous to provide a robust, cost effective and high-quality system for this kind of engine.